In photolithography, a desired pattern is applied onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material, usually referred to as a resist, which is provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured. A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. Such radiation is sometimes termed soft x-ray radiation. EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. Such a radiation system is typically termed a laser produced plasma (LPP) source. Alternative sources include discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
A particular challenge for the development of commercial EUV lithography lies in the formulation of radiation-sensitive resist compositions that will realize, in etch-resistant material, a high resolution pattern projected by the EUV optical system. A photoresist typically converts an optical contrast to a chemical contrast that can be developed to transfer a pattern into resist. For EUV lithography, the state of the art photoresists are chemically amplified photoresists (CAR) which have a trade-off between resolution, line width roughness and sensitivity. Due to the interdependence of these properties, a modification made in a photoresist composition to improve one of these properties usually results in deterioration of one or both of the other properties. Another challenge is that the manufacturing of such resist compositions may need a synthesis step at a high temperature or it may need to use non-radiative processes to reduce the electron —ligand efficiency.
During exposure, the EUV photons are absorbed by the photoresist. Due to the high energy of the EUV photons, absorption by the core levels of the atoms is the main mechanism and therefore the atomic composition and density of the photoresist are the main drivers for EUV absorption. Current organic photoresists all have a similar EUV absorption coefficient which results in about 25% of EUV absorption, the rest of the photons incident onto the wafer is mainly wasted. The total absorption is limited by the side wall angle considerations which are related to absorptivity and the thickness of the resist layer.
During exposure photoacids are generated from photoacid generators (PAGs), which further catalyze a deprotection reaction at the post exposure bake step applied to the photoresist. When a certain amount of deprotection ratio is reached, that part of the photoresist can be dissolved away in a development step. In this way a resist pattern with certain dimensions and roughness is formed.
The diffusion during post exposure bake is generally random in all directions and forms along with the secondary electron blur an important part of the total blur. Limiting the horizontal diffusion of acids and promoting vertical acid diffusion (in z direction) would be beneficial. A resist would ideally show a limited blurring in x-y direction and predominant blurring in the z direction, the photoacids diffusing along the resist height (z direction) will deprotect a larger volume than if they diffuse isotropically.
The current organic based chemically amplified resists generally don't meet all the requirements such as dose, roughness and resolution requirements for features smaller than 16 nm for EUV lithography. The generally low absorptivity of CAR results in a limited sensitivity of the resist, leading to high doses being required to mitigate the effect of photon shot noise at small pitches.
Increasing the total absorption would be beneficial in order to use the EUV photons more efficiently. However increasing the total absorption while keeping the resist profile at about 90 degrees is still a challenge. If higher absorbing materials are used then the total resist film thickness needs to be adjusted/reduced accordingly, at which point the etch resistance becomes a problem. Another problem is pattern collapse, which limits reaching high resolution and high aspect ratio materials. Another cause of the pattern collapse is a poor adhesion of the photoresist to the substrate and a low mechanical strength of the resist material. There is a need for new resist compositions which solve one of more of the above problems.